Johnston noticed that some patients suffering from heroin overdoses needed larger doses of Narcan, a drug used to reverse the effects of an overdose, to help save their lives.

"We started to see a change in how the reaction to the Narcan was going," said Johnston, station commander at American Medical Response in Brockton. "Sometimes it was delayed."

Johnston believes the delay is due to a drug called fentanyl – a synthetic painkiller 100 times more potent than heroin, often prescribed to terminally ill cancer patients. When mixed with heroin, the combination can be deadly, experts say.

Dr. Joseph Shrand, medical director of CASTLE (Clean and Sober Teens Living Empowered), part of the High Point Treatment Center in Brockton, said those who use heroin should be extra cautious about what's out on the street.

"It's a very scary, dangerous thing right now," Shrand said. "When you buy something on the street, it's not FDA approved. You don't know what the heck is in it. There was a similar situation with Molly (MDMA) lately."

Johnston believes fentanyl is one of the reasons the ambulance service has seen more than double the number of heroin overdoses in recent months.

"In a week, lately we've seen an average of about 6-8 overdoses. Normally it's 2-4," he said.

Dr. David Rosenbloom, professor of health and policy management at Boston University, said the drug is far more potent than heroin, which may cause a user to unknowingly take more than they are used to.

"The problem is that it is so much more powerful than heroin that people who take mixtures of it are at significantly greater risk for overdose," Rosenbloom said. "Fentanyl slows down breathing and if you have enough of it, then you stop breathing."

In Pennsylvania there were at least 17 fatal overdoses in January where the deadly concoction was believed to have been used, according to ABCNews.com. In Maryland at least 37 deaths were linked to the mixture since September, the website said.

Massachusetts State Police said they are gathering statewide information to figure out how to approach the issue.

Massachusetts State Police Lt. Daniel Richard said they are looking to see if fentanyl is being used in one particular region or statewide. He also said the state drug lab is reviewing the heroin they have recently seized for traces of fentanyl.

Locally, Easton Police Chief Allen Krajcik said he would not be surprised if fentanyl has made its way to Massachusetts.

"If it's being used elsewhere, then I'm sure that people buying it in Easton may be getting it in their heroin," Krajcik said.

The drug is prescribed in numerous fashions, as a pill, powder or liquid. Some of the other more common ways it is prescribed is as a patch or lollypop form.

Scott Peterson, an employee at Learn2Cope, a support group for family members of addicts, said he believes some people are breaking open the patches to expose a jelly-like substance that contains the medicine.

He said the only way people can get fentanyl is with a doctor's prescription.

CASTLE's Shrand urges people who are going to use heroin to not shoot up alone. He said if someone does begin to overdose, to call 911 immediately. He also said that users should keep handy a dose of Narcan – the brand name of the drug Naloxone – for when overdose situations arise.